A comparative ecophysiological study was carried out in September and November 2002 for two bivalves, Paphia rhomboides and Glycymeris glycymeris, which are significantly harvested in the Western Channel (French coast). Individual measurements of clearance and respiration rates were performed for both species, in several experimental conditions of temperature and phytoplankton concentrations. Those conditions were chosen to be in the range of those commonly observed in the subtidal area of the Western Channel. The mean value of clearance rate and oxygen consumption obtained from continuous data recording did not reveal any significant influence of temperature in the investigated range, but a significant difference between P rhomboides and G. glycymeris.

P rhomboides showed a continuous feeding activity and faeces production and was characterised by high clearance rate (mean = 1.7 1 h(-1) g(-1)) regulated by the phytoplankton concentration and high assimilation efficiency (approximately 90%). On the opposite, G. glycymeris showed a periodic feeding activity and faeces production with low clearance rates independent of phytoplankton concentration (mean = 0.4 1 h(-1) g(-1)) and a lower assimilation efficiency (approximately 78%). Individual SFG values were calculated for both species and it appears that P rhomboides exhibited higher SFG values than G. glycymeris, except for low concentration of algae. These results may help to explain the significant difference in growth rate of both species. However, assumptions are made on differences in food source and/or feeding behaviour of P rhomboides and G. glycymeris.